We are a team of two and we both agree that our best cache find this year was "Le courrier du géocacheur" in the town of La Redorte in the Languedoc in France (GC6FMDK). It was a letterbox cache with a difference - it was an actual 'letterbox' facility! If you wished, you could leave a postcard with your address and a postage stamp in the box (there were some there to use if you hadn't taken one of your own). You left the message part blank. The next cacher was then asked to put your card in the real french postbox nearby, after having written you a message. The card would then hopefully be delivered to you after you had returned home.
We picked up the postcard that was already there and posted it to a cacher in Germany. We left one we had taken specially (having read the cache description in advance) and eventually it got picked up and sent by another french cacher to us in the UK.
We stamped our notebook with the letterbox stamp and left our stamp in the log! It was a great idea and a memorable caching experience, and it worked!